1. Field of the Invention
This invention relates generally to an apparatus and method for fabricating directional optical taps, sensors and other devices with variable angle tap outputs into optical fibers and to the optical taps themselves. This invention also relates to a method and apparatus for making optical sensors, for example, sensors for determining optical fiber misalignment losses with other optical devices, strain losses in optical fibers and optical mode filters. This invention also relates to a method and apparatus for measuring position of an object and refractive index of liquids.
2. Background of the Related Art
Optical fibers are replacing wires in telecommunication systems, data link systems such as local area networks (LANs) and sensor systems such as "fly-by-light" systems in aircraft. Optical fibers are advantageous in these systems because fibers can carry significantly more information than electrical wires. For example, an optical fiber can carry up to 5 billion bits per second whereas a wire can carry a maximum of 150 million bps. In addition, optical fibers are reliable and more compact than the electrical wires they replace. Also, optical fibers do not produce electro-magnetic interferences nor are the signals they transmit effected by electro-magnetic disturbances.
Losses are encountered in any couplings between an optical device (e.g. a laser, an optical modulator, an optical waveguide etc . . . ) and a fiber or between two fibers. Installation and upkeep of these couplings represents a costly aspect of fiber optic telecommunication systems, fiber optic LANs and fiber optic sensor systems. One reason for this is that installation of optical devices must be done by hand using specially trained and expensive technicians. It is therefore desirable to be able to conveniently and inexpensively couple information from an optical device or fiber to another optical fiber.
Another problem with current optical couplings is that they can produce significant signal losses. One reason for this is that optical fibers have very small fiber cores, (the diameter of fiber core is typically between a few micrometers to a several hundred micrometers) making it difficult to align and maintain alignment between optical devices and fibers or between two fibers. It is even more difficult to couple a bundle of optical fibers contained in optical cables with other optical devices or another bundle of fibers. It is therefore desirable to be able to conveniently and non-intrusively determine whether the coupling is producing losses insuring minimal signal loss at the connector.
In addition to coupling losses which occur at couplings between optical devices and fibers, additional losses can occur due to bending or straining optical fibers. In such situations, a significant amount of radiation is lost due to guided cladding modes and radiation (leaky) modes.
Optical time domain reflectometry (OTDR) has been used as a distributed reflective loss monitor in such optical data link systems. OTDR systems involve sending a pulse of monochromatic light of a known power level into one end of a particular fiber link in the fiber optic system and measuring the reflected power level due to discontinuities such as splices, connectors or fiber breaks. This approach does not however locate the strain or misalignment in short distances or in complicated local area networks. Also, this type of OTDR requires significant additional complicated components as well as access to the end of the fiber in order to launch the laser pulses into the fiber.
Another technique for status monitoring in optical fibers involves using clip-on optical components or sandwiching optical fiber between a grating and a lens. These clip-on optical components couple a small amount of laser light out of the fiber core. However, such clip-on components are not suitable for long-term installation because they place a high stress on the optical fiber which can generate microcracks. Consequently, it is advantageous to have a simple, non-intrusive, long-term technique to determine whether strains and misalignment are being placed on optical fibers in any type of fiber system without disconnecting any fibers in the system.
In addition to providing a convenient and inexpensive approach to minimizing losses due to couplings, it is desirable to replace electrical sensing systems such as electrical position sensing systems with optical sensing systems such as optical position sensor systems. Fiber optic sensing systems offer numerous advantages over conventional electrical sensing systems because they are small and light weight. It is even more advantageous if sensors in the sensing system are completely passive optical components, i.e., they have no active electro-optic components such as semiconductor lasers or light emitting diodes. Passive sensors are desirable because they would be immune from electromagnetic interference (EMI) which occurs near power lines and they would also be immune from electromagnetic pulses (EMP) which can occur in the event of a nuclear explosion. EMI/EMP immunity is especially important advantage for new generation aircraft which have skins made of composite materials.
Smart skin technology relies on embedding a plurality of passive optical sensors in a structure such as an aircraft wing. Smart skin technology can be used to passively measure strain or other local deviations of parameters such as strains or temperatures with a particular spacial resolution, wherein a large number of optical sensors produces a high spacial resolution. Consequently, it is desirable to be able to fabricate optical fiber with a large number of optical sensors, which can be used in smart screen structures.